Method and apparatus for synchronizing oscillators



R. C. MOORE May 9, 1950 METHOD AND APPARATUS FOR SYNCHRONIZINGOSCILLATORS 5 Sheets-Sheet 1 Filed Sept. 25, 1944 May 9, R C, MOOREMETHOD AND APPARATUS FOR SYNCHRONIZING OSCILLATORS Filed Sept. 23, 19443 Sheets-Sheet 2 BEA-r FREQ.

IN VEN TOR.

May 9, 195o R, C MOORE 2,507,317

METHOD AND APPARATUS FOR SYNCHRGNIZINC OSCILLATORS Filed Sept. 23, 19443 Sheets-Sheet 5 AGG/N6 O LEAD/NG PHASE /NCREMENT UPPE l? REFERENCEREFERENCE CRKSTA I. CRYSTAL OSC/LATOR AND CONTROL IN VEN TOR.

Patented May 9, l1,95@

METHOD AND APPARATUS FOR SYNCHRONIZING OSCILLATORS Robert C. Moore,Philadelphia, Pa., assignor, by

mesne assignments, to Philco Corporation, Philadelphia, Pa., acorporation of Pennsylvania Application September 23, 1944, Serial No.555,528

9 Claims.

. kAn object of the invention is to provide a system for synchronizingtwo oscillators at different, but related frequencies.

Another object is to provide a standard signal generator of highaccuracy, which is adjustable in frequency, and which may be operatedover a wide range of frequencies varied in small steps.

A further object of the invention is to provide improved methods forsynchronizing oscillatory apparatus and for controlling frequency.

Further objects of the invention are to provide methods and apparatusfor synchronizing a tunable oscillator with a crystal of differentfrequency.

Other and further objects, features and advantages will become apparentas the description proceeds.

In carrying out the invention in accordance with one embodiment thereof,a crystal oscillator serving as a reference frequency is provided, andsignals from the crystal reference are mixed in a mixing device withsignals from a local oscillatorwhich is tunable and which is to tbecontrolled. The mixing device is utilized to give an outp-ut of thedifference or beatfrequency. The crystal signal is also divided down toa lower accurate frequency and formed into a pulse. The pulse signal andthe beat signal are fed to a balanced discriminator circuit. In thismanner, a direct current voltage is developed by the discriminatordepending on the phase relationship between the beat-frequency signaland whichever multiple of the accurate pulse signal corresponds infrequency. The direct current voltage is applied to a reactance tube tokeep the tunable local oscillator at such a frequency and phase that thesystem is kept in equilibrium. By this means the frequency and also thephase of the tunable oscillator are kept in strict relationship to thecrystal reference.

A better understanding of the invention will be afforded by thefollowing detailed description considered in conjunction with theaccompanying drawings, and its scope will be set forth in the claims.

In the drawing Fig. 1 is a schematic block diagram of an embodiment ofthe invention.

, Fig. 2 is a circuit diagram of a portion of the apparatusof Fig. 1. l

Figs. 3, 4 and 5are graphs explanatory of the 2 principleV of operationof a portion of the apparatus.

Fig. 6 is agraph illustrating the principle of operation with adifferent frequency-step adjustment from that represented by Figs. 3 and4.

Fig. '7 is a diagram representing the screen of the monitor tuberepresented schematically in Fig. l.

Fig. 8 is a schematic block diagram Vof an arfrangement in whichreference frequencies are employed having an odd ratio or having suchvalues that a division corresponding to the ratio cannotV convenientlybe carried out electrically.

The arrangement'illustratedin Fig. 1 comprises a frequency standardrepresented by a rectangle Il, an oscillator and control unitrepresented by a rectangle l2, anda cathode ray tube type of monitorrepresented by a rectangle I3. The frequency standard is preferably of atype which provides output at two different frequencies. If desired, afrequency multiplier I4 may be provided for enabling frequencies to beproduced beyond the order of magnitudes readily provided by theoperation ofthe control unit l2.

For enabling the frequency standard to be compared with an absolutestandard such as the signal supplied by the United States Bureau ofStandards, and transmitted` from aradio station such as vstation WWV. anamplifier l5 may be provided for supplying signals from station WWV tothe frequency standard.

AThe frequency standard Il comprises a reference crystal oscillator Il,-whichfor the sake of illustration is represented as aIDO-kilocyclecrystal oscillator, a pair of frequency changing devices |8and i9, which in the specific case illustrated, area frequency dividerand a frequency multiplier respectively, and a beat-frequency detectorand amplifier 2|, supplying a detector such as head-phones I6 forrevealing any discrepancy between the output of the 'frequency standardilV and vanv absolute standard such as the signal from lthe WWVamplifier I5. The frequency changing devices i8 and i9 are provided inorder that a single reference crystal Il may be utilized for causing twodifferent reference, control or standard frequencies to be pro# duced,one of which is approximately the frequency of the tunable oscillator tobe controlled, namely an upper reference frequency, and the other ofwhich isa lower reference frequencyor incremental-frequency. For thesake of illustration, it is assumed that the oscillator to becontrolledhas'aV frequencyof 1 megacycle, and that the incremental frequency is tobe 10 kilocycles,

In this case the frequency divider I8 may consist of two steps 22 and 23h-aving division ratios of 2 and 5 respectively, and the frequencymultiplier I9 may also consist of two steps 24 and 25 having frequencymultiplication factors of 2 and 5 respectively. The invention is notlimited to the use of frequency ,changing devices'of any particulartype. However, it has: been found satisfactory in the case of thefrequency divider I8 to employ blocking oscillators for the two steps 22and 23, and in the case of the frequency multiplier I9 to employ vacuumtube"ainplilf1ers"tvith tank circuits tuned to the desired harmonic. y

The frequency standardjur'l'it llfis provided with two output channels26;-ad21gf'supplied through the frequency divider "I3"and"the`"'frequency multiplier I9, respectively; from thereference crystal I'I.

"acta'nc'e tube -"34 connected'across the tank circuit of the oscillator28. The beat frequency 'mixer 32 is so arranged as to mix the output ofthe beat frequency detector 42 differentially with l the output'of thepulser 3l.

For convenience of description and explana- "tion specific illustrativenumerical values of fre- The oscillator and control unit I2 cornrisesf-alocal or tunable oscillator 28, a high frequency mixer 4andlbeatproducing device `29, which in" the specific case illustrated takes theform of anin- `-d uc'tive 'miXerQa puiser 3 I g abeat frequency mixer32,'which in theV spe'cc' case illustrated takes the form of aresistance mixer, a discriminator 33g`a frequency adjusting element inthe 'form of a'reactance tube 34 connected in controlling relation tothe local oscillator 28, and a suitable output 'terminal 30 connectedto' the oscillator 28. The oscillator and' 'control'unt I2 is providedwith two input connections represented by the output channels 26 and 2Tof the frequency standard Il connected respectively to'the pulser 3l andtheV high frequency mixerorbeater 29.

If frequency multiplicationis desired, the" frequency multiplierffM maybe 'connected to' the output terminal 3|) of the oscillator and controlunit I2.

The cathode ray tube `monitorIl'comprises a cathode ra'y indicating`tube 35, having conventional deflection and' control circuits with' a10 kilocycle amplifier 36 for `supplying deflection .voltage `ofadequate magnitude from the 10 kilocycle output line 26`of the frequency'standard I I, ap'a'ir' of phase Shifters 31 and 38 for converting theoutput of the 10kilocycle'amplier`36 into a pair of quadrature voltages,and a'beat-frequency amplifier 39 forv supplying the output of the 'highfrequency mixer or beater`29 tothe control` circuit of thecathode raytube`35.

The cathode ray tube include'san"e1ectron gun having a `controlgrid-35ain"addition to other" conventional elements `of such' apparatus andthe deflection circuits. 'The latter are representedbypairs ofelectrostatic deiiection plates 35h and 35C respectively. The plates 35hare `con 'nected' to the phase shifter l'31,- the plates Y35o areAconnected to the phase shifter( 38,-and the Vcontrol grid 35a isconnected to the beat frequency amplifier 39. It is to be understoodthat the Vcon 'nections are represented only schematically, not beingcomplete electric circuit connectionsfand 'that certain conventionalelements have been omitted.

and shown'in more detail'in Fig. 2; comprises the oscillator 28, acathode-follower tube 40 interposed Abetween the oscillator 28' and theoutput terminal 30 'for isolatin'g'the oscillator `28"frornthe outputterminalland'falso isolatingit "from the he apparatus represented'bytlierectangle 'I2 `grid Colpitts type.

Yfquen'cy have been mentioned. It will be understood, however, that theinvention is not limited *tov these valuesor to the ratios between them.

It willube understood likewise that the inventio'nfis not limited to theuse of a particular type or form of oscillator oralternating-currenilgenerator. "For the sake of illustration the tunable oscillator 28 -hasbeen represented as a vacuum tube circuitA of tlie--series-connectedgrounded- The oscillator as illustrated comprises a triode vacuum tube4l having the conventional anode 48,-cathode 49 and'control electrode or'grid 5l.

The circuit as shown in the particular arrangement illustratedutilizes-a gridl grounded with re- Vspecixtoradio frequency,'andaccordingly-afgrid resistor `52 is provided which` is .by-passed byc'acondenser 53. A radio frequency 'choke 54 -isconriected in series withrthe cathodeV 49-and aground connection, andV 'aA ysuitable ysource ofanode Jor plate voltage, represented by a; Bbattery 55; is providedhaving a positiveterminal connected to the anode-48 and having aAgrounded negative terminal. Other powersupply connections nare, forconvenience, indicated by positive terminals with'the same referencenumeral 56.

v`The vacuum'tube 4'I is'provided with a tunable tank circuit consistingof an inductance 5ln shunted -by an adjustable condenser 58 serving topermit adjustment of the rfrequency -of the oscillator 28. Theinductance 5I-is' connected in series with the plate lead. lAconventional 'decoupling network comprising a. seriesfresistor59and a:condenser 6I Amay be' utilized.

"Forcorrecting the actual frequency of the output' of theoscillator 28,the-reactance tube=34 is connected across the tank circuit consisting ofvthe inductance 57 and the condenser 58. In order toenable thereactance'A tube 34' to l'exert' a 'greater degree; of "controlton thefrequency ofthe oscillatorv28; the oscillator Vtubeill'i is'preferablyoperated ata lower'f plate' voltage 'than the vreact- 'aricetube 34.Leccordingly, a dropping resistor '62 is "also 'connected in Vseriesbetween the'ianode 48' 'and thepositive terminal/56 of the power'supply,and theresistor 62 is vby-pa'ssed for radio fre- :quencyby acondenser63.

It will belund'erstood that'as in the 'usualffColpitts circuit,feed-'back coupling ccnd'ensers 64 and '65, connected ini series acrossthe tank' circuit are utilized with their junction terminalcon-`'nectevd to the'c'athode 49.

`The reactance `tube`34may`take the forni of a pentode or other'appropriate Avacuur'n tubes'having conventional 'electrodes inciudingan'fanode 61, a cathode 68 and a control electrode orgrid et. Thecathode sajis shown as'cormectedm'series with a by-'passedres'istorl'lfo1"prov`iding normal cathode bias. The anode B'I is connected tothehigh potential 4terminal '318 fof 'thetank circuit 5l-`58^ofthe'foscillator 28. For 'produc- Uments 98 and 99 with load resistors.

ing the reactance effect, the control grid 69 is coupled to the tankcircuit 51-58 through a phase shifting circuit consisting of a condenser12 and a resistor 13 in series.

As will be explained in greater detail hereinafter, a connection is madefrom the discriminator 33 through a conductor 14 to the grid 69 for.varying the bias of the tube 34 and thereby varying the reactanceeffect and the tuning of the :oscillator 28 in response to variations inthe direct current output of the discriminator 33. Preferably asmoothing circuit comprising a series resistor 15, and a filtercondenser 16 is connected between the conductor 14 and the terminal 11of the resistor 13 forming a part of the lphase shifting network.

The high frequency mixer 29 is illustrated as .taking the form of atransformer 18 having a primary winding 19 coupled to the one megacyclechannel 21 through the buffer stage 4|, and having a secondary winding8| conductively connected to the output terminal 30 of the oscillator28. The cathode follower 40 serves to couple the tank circuit 51-58 tothe output terminal 30.

, Preferably, an adjustable tuning condenser 82 is connected across thesecondary winding 8| of vthe transformer 18 to enable the winding 8| tobe tuned broadly to the frequency of the oscillator/ZB and the referencesignal supplied from the channel 21. To avoid excessive sharpness oftuning, a resistor 83 may also be connected across the winding 8|.

n The beat frequency detector 42 may take the form of a diode or adiode-connected triode, as

`shown, with a load circuit comprising a resistor 84 in series with achoke inductance 85, to improve the response at higher beat frequencies.

The beat frequency mixer 32 is represented as being of the differentialtype having two outputs, one combining the inputs in opposite relation.from the other output. As illustrated, the beat frequency mixer 32comprises a pair of resistors 86 and 81 coupled respectively to one ofthe outputs of the phase splitter 44, and the output of the pulser 3|,and a second pair of resistors 88 l`and B9 coupled respectively to thesecond output .of the phase splitter 44 and the output of the In orderto provide balance adjustment, a re- V:sistor 9| with a groundedadjustable tap 92 may be connected between the junction terminal 93 'of.the resistors 86 and 81 and the junction terminal 94 of the resistors88 and 89. It will be understood that the phase splitter 44 in the form'illustrated comprises a vacuum tube 95 with anode and cathode loadresistors 96 and 91, with an input coupling from the beat frequencydetector 42, and with anode and cathode output couplings respectively tothe resistor B6 and the-v resistor 88.

The junction terminals 93 and 94 of the mixer -32 serve as outputterminals and may be coupled directly to the phase discriminator 33, butpreferably amplifiers 45 and 49 are interposed in the coupling.

The phase discriminator 33 comprises a pair of diodes or otherunilaterally conducting ele- A bridge arrangement may be employedcomprising re- `sistors |0| and |02 connected in series between `diodeanodes and having a junction terminal |03, and resistors |04 and |05 inseries between diode cathodes with a junction terminal |06 connected tothe terminal |03. A smoothing condenser. |01 is preferably, connectedacross the resistors |04 and |05.

The invention is not ylimited to utilizing a particular form of pulserfor converting the incremental frequency input, in this case a 10 kilo--cycle signal from channel 26, into short duration pulses. For the sakeof illustration, however, the pulser 3| has been shown as taking theform of a single pulse or triggered multivibrator circuit. Asillustrated, the pulser 3l comprises a pair of discharge tubes such astriode vacuum tubes |08 and |09 having a common cathode resistor fornormally biasing the tube |08.to cut-off. The tube |06 is ,provided witha grid biased to ground while the tube `|09 is provided with a gridbiased to cathode p0- tential, whereby the tube |09 is normally in a.current conducting condition. A conventional capacity-resistancecoupling represented by a condenser I2 and a resistor |3 is providedfrom the tube |08 to the tube |09. The common cathode resistor serves asa coupling from the tube |09 back to the tube |08.

For providing an output of adjustable amplitude from the pulser 3|, anadjustable tap |l4 is provided on the cathode resistor and is connectedby means of a conductor ||5 to the resistors 8l' and 89 of the beatfrequency mixer 32. As previously explained, the buffer tube 43 ispreferably interposed between the 10 kilocycle input channel 26 and thepulser 3 being coupled to the input of the tube |08 of themulti-vibrator pulser 3|.

Since the oscillator 28 and the reactance tube 34 are conventionalelements, it will be unnecessary to describe the manner of operationthereof in detail. Nevertheless, it may be pointed out that thereactance tube 34 is coupled to the tank circuits 51-58 in such a mannerthat it draws a quadrature current, the amplitude of which depends uponthe potential to which the grid 69 is biased by the output of thediscriminator 33. The quadrature current drawn by the reactance tube 34acts as the equivalent of a variation in the reactance of the elements51-58 of the oscillator tank circuit. Accordingly, the

direct current output of the discriminator 33 determines the frequencyof the oscillator 28.

The constants are preferably so chosen that adjustment of the condenser58 or another suitable tank element provides a rough adjustment in thefrequency of the oscillator 28 and changes `its frequency from one stepto another, whereas the control ofthe reactance tube 34 provides a fineadjustment. In the case of the particular numerical values assumed forthe sake of illustration, the frequency of the oscillator 28 may `bevaried in 10 kilocycle steps above or below a mid-frequency of athousand kilocycles. To change the frequency from one step to another,the condenser 58 is adjusted. When the frequency deviates from one ofthe predetermined vsteps such as 1000 kilocycles, plus or minus 10kilocycles, 20 kilocycles, etc., the .apparatus automatically varies theadjustment of the reactance tube 34 to restore oscillator frequency tothe exact value of one of the 10-kilocycle steps.

In case the frequency of the oscillator 28 differs from the upperreference frequency provided by the frequency standard through thechannel 21, a beat frequency may be detected by the detector 42. Thewave form of the frequency standard output channel 21 .is representedschematically in Fig. 2 by the series of sine waves H6. If theoscillator 28 is adjusted example, a beat frequencyy signal. of, 10kiloeycles wilfll.' be produced V `in lAllie u detector: 42., wliich isarepresented-schematicallyby the sine waves. lll. '-ltmay beobservedthat the .tuning of .themixer wmdirsg 8lr tor'approximatelyl 1.rmegacycleV by .the condenser. tserves tomllter out harmonics.parasitics anoto eliminate Yfrom. .the `detector output .anybeatrequencieslexoept .the one rep- -resentedlbyithe-adierence between-.thereference :frequencyllleandthezfandamental ware ofthe output`of..the:oscillator 2o.

. In; the particnilarf linstance assumed, the beat ifi-equency 1 I H has.theo sameirequency .value as the :incremental-reference frequency, inthis. case -a 10-kilocycle-wave-supplied through the channel`2Iby-1tl'ie.` frequency standard l l and. represented `A irrliigloy thewavel I8.

f The-sconstants ofthe puiser 3l. are sovchosen Y -thai1the on-to ratioof thetubes .m8 anclJllSl visveryhigl:1., and pulses ofi-short. durationArepresented bythe wave formllS .are produced at the output line H5ofthe same frequenti;7 as thein- `@semental or lowenreferenoe-frequencywave H8. `Auwill .become apparent as` the description-.pro- -.eeeds, thevnumber of-.dii'ferentsteps of frequency, -zwhich may becontrolled bytheapparatus, is cletermined Vby' narrowness Yor .shortness of duration of.thefpulsesJ lproduced by the multi-vibrator 3L #Althoughtheexact limitYorelationship be.-

.-.tweenirequency rangeand the narrowness of thenpulsesll 9 has notbeen.denitelyasoertained, it has been found that where the frequencyrange is.to bla-200l kilocyoles; for example, above or .below Ythevtmid-irecmency,f satisfactory .operation is obvtained Withrpulseshaving, the.durationl of microseconds. That isto say, the pulsedurationlcorrespends to the period of a .cycle oi .the maximum.beatifrequerncy which iszto oe-encountered. 1 When ,the oscillator. 28iis. tuned.. bymeans ofthe condenser. atothe .frequency oflzokilocycles, the ibeatirequencywith the -..reference .wave H6 at 1megacycle is 200.kilocycles.

.The phase. detectorV action of. the discriminator 13G-isexplained-graphically.by-,thecurves of Figs. f3,..top6;w.'l"he1. action.oil-thev vcliscriminator 33 is responding. to- .phase Arelationship.between the pulse warme` l' t9 :andthebeat frequency wave-if ilfexplainedforthe particular ,caselwhere `.the beat frequencyequalsthapulse frequency by the `graphstfi-liigs. 3 and 4, Preferably,theconstants 1of;the.appara--tus.are so chosen that the voltage swingothereetangular wave H9 is as great ras thepeakivalue of thebeatfrequency. wave H1. .Referring co-Fig. 3, it' is to be observed that'when .the yreetanerular .wave M9 andthe `beat frequency WavelHaresuperimposed, awaveflll is l.produced :having av peak-value greatervthan that .of either. wave, andthemagnithde of whichde- ...pendelupontlie phase relationship. between the Waves Hland- H9. VFor example,the-,.peak.. Value iothewave l2 Iv is representedby thedistance 122.'The mixer 32combines the waves 1li?. andfHQ ;aa4 iditively.inthediscriminator diode 89,.- and `coin- ;binesthemdiierentiallm with the.beat frequency fzwave reversectin the discriminator .diode S3. Thevoltage applied tothe .discriminator diodel S9, is representedfby thewave i2! of Fig. 3. Ori-the a'otherhanmthe yoltage applied to thediscrimina- .:tor :diode-i538 is-represented by a'voltage |23, :whichisa resultant oie rectangular pulse I i9 fand a reversed sine `wave H1.Consequently, the peakwalueoiZ-the wave-.1.23 is lower asrepresented-by.the-distance l24.v The rectijlers ...andrause'-currermtoow through theresistors i185.; and; III:- respectively, `which; .are proportional .tothe peakzvalues .MI andetzll. otwaues tzt and .-.t2&..respectiue1y;,Y.Ancordiuglssf, .a .Voltage diierence appears acrossethe; endsyo,.tlz1e-.resistors L04 and; :Lil 5; .corresponding .to .the dlerenceHibe- Vtween: .thepeak values of .theiwaves 1.2L andeli. .Suchaviolilage .is storedsfon vtlfle--condmaser still and serves. to adjust.the .bias oithe reactanoeitube :3A in.A maler` toradjnst thenaturalfrequency of -the-,tanlscircuits 5l=58 of. theoscillaton'.: TheYvariation ,qua-droiture.. current.. drawn.v in parallel with..tln.e..tanlY circuits 5l+58 serves .tofadjust the eiectivephase or theoutput or the-oscillator--28 soil-aat theVV beat-frequency .Wave -Mfliseither-advanced or retardecluntil the -diierence between peakvaluesfoithe waves .12k-and $23 @allstol`r zero, as. illustrated by thecondition ofUFig. A4 where-in.the peak of-.th-e rectangular wave H9occurs at-the-zero .of-"the beati-fliequency wave `Ml'..

. Thevariation msoutput oli the-discriminatori with variation inphasebetween therectanlgular lwaveii and its corresponding wave itl-is-.rep-

`resented-.by the. curve in. Fig. 5A inawhich theldrrect current outputof the discriminatonflt-is measured vertically, and lphase.angle-leading or .lagging is measured .a .horizontal directionTheefireuuency... of.` the-oscillator ilmayrdifer by ymore than. oncethe .incremental reference frequency from Vthe upper referenceLfrequency .if there isar diilerence in .phase between thebeat frequency-wavel-lalAaand a. multiple ofthe' rectangular. v wave frequency. rl' l.El, correspondinjgzsto .the .zrequency off theiwave rt I 1A, there.Ywill.A bea difference in the peak values of resultant-Waves 421'..anclT 123', Vand the diierence of peak- Value represented :by thedistance |25'. willcproducev. a current givingrise to aunidirectionalvoltageoutfputapplied .through .the Vconductor 'i4- to thegrid vcircuit.of fthenreactance; tube. 3i Accordingly, the phase ofA the oscillator;28 .is adjustedauto- .matically to correspondexactly to thepl'iasefol`the reierenoe. wave` -I t6,` although not. at the same `frequerwy,

Whenthelocaloscillator 28isoperating atsub- Y.stantially-- thefrequency.ofqtheupper reference wave H6 supplied by the channel 2l,variations ,from exact5 phase coincidence `between the output of. theoscillator 2.8 and .thereference-Wave -ih ',wiil.. be ,indicate-d by..the output of thesdisfcriminatorz, .and-the polarity of` thediscriminator 33 will depend upon the sign of hephase angle; if.. ,cwhetherthe phase variation is lagging orleadmgasrepresented bywFig.`v 5.Y :Similarly .when the oscillator-2S is .operating at diierent -setfrequencies above or below therequencyfpf the upper freference gl, i6,the A polarity of gthe out- .putor l.the discriminator.; 33 will vdependYwhether ther apparatus ,is operating above or be- .1o-w,` aireauerxrdeviatinsa xed numberffsrs n oom $1.152.- rsieren freaueoyf-fhe@sconfinata 33 acting through the reactance tube 34 will thereforecorrect the frequency for any frequency setting of the condenser 58.

The operator of the apparatus may ascertain from the cathode-ray-tubemonitor I3 of Fig, 1 Whether the oscillator 25 has locked in at thereference frequency I I6 or at some other step frequency; and maydetermine the step in which.

the oscillator has locked in by observing the screen of the cathode-rayoscilloscope 35, referring to Figs l. and '7. It is to be observed thatthe oscilloscope 35 has a sweep circuit energized at the frequency ofthe incremental reference wave, in this case a 10 kilocycle frequency,causing the cathode ray beam to rotate at this frequency. The grid 35aof the cathode ray tube, however, is energized at a frequency equallingthe beat frequency or difference'in frequencies between the upperreference wave II6 supplied over the channel 21 and the frequency of theoscillator 28. Accordingly, the potential of the grid 35a will be raisedone or more .times during each rotation of Vthe cathode ray beam, thenumber of times equalling the ratio of the beat frequency to theincremental reference Wave frequency, which latter is the sweepfrequency of the beam. For example, with apparatus having the numericalvalues assumed for the sake of illustration, when the oscillator 28 isoperating at a frequency of 1040 kilocycles or 960,-kilocycles, the beatfrequency will be 40 kilocycles or four times the frequency at which thecathode ray beam is rotated. For this condition, as illustrated in Fig.'7, four spots E25 will be produced in a screen'of the cathode ray tube35. The spots I26 will be light spots or dark spots depending on whetherthe grid 35a is so biased as to cut-off the cathode ray tube exceptduring the peak values of the output of thebeat frequency amplifier 39or to cut olf the tube only during the minimum values of the beatfrequency amplifier 39. The actual length of the spots will, of course,depend upon thev magnif tude of the bias of the grid 35a of the cathoderay tube 35. It will be understood, of course, that if desired the beatfrequency amplifier 39 maybe provided with a peak wave output so as tosharpen the spots.

Whether the oscillator 28 is operating at a frequency above or belowthat of the reference Wave IIS, may readily be determined bymanipulation of the frequency adjusting condenser 58. For example, ifthe condenser 58 is turned toward the position increasing the capacitythereof so as to decrease the frequency of the oscillator 28, butobservation indicates that the number of spots IZB of the screen of thecathode ray tube 35 is' increasing, it is apparent that the oscillator28 is operating at a frequency below the.' mid-band frequencycorresponding to the' frequency of the upper'reference wave IIB.

The'number of spots |26 seen on the screen of the cathode ray tuberepresents the frequency deviation of the oscillator 28 from thereference wave Ill;` expressed as a multiple of the incrementalreference frequency IIB. The term multiple is used in the specificationand claims- 10 other and that the relationship is such that the divisionor multiplication may readily be carried out by electrical means.However, the invention is not limited to the speciflc arrangement ofFig. 1 in which a single crystal reference I'I may provide two referencefrequencies by two different frequency changes or by a single frequencyvchange and a direct output from the crystal reference to one of theoutput channels. For example, if the desired 'output frequency or thedesired frequency steps 'in the variation of the oscillator `frequencyshould be such as not to adapt themselves to electrical change of asingle reference frequency to two different reference frequencies, twoseparate crystal references or other standards may be utilized.

As illustrated in Fig. 8, a crystal oscillator or a source of arbitrarystandard of frequency represented by a rectangle S28 is utilized toprovidel the upper reference frequency. A second lower referencefrequency source represented by rectangle |29 may be utilized to providethe incremental reference frequency. The outputs from the two referencefrequency sources |23 and |29 may be supplied through the outputchannels 26 and 2'! for the operation of the control unit I2 asdescribed in connection with the apparatus of Fig. 1 and Fig. 2.

It is to be understood that the frequency standard I I is checkedagainst an absolute standard such as theoutput of the Bureau ofStandards radio station WWV `by comparing the output of the WWVamplifier I6 with the frequency of the crystal reference I1 or oney ofits harmonics by means of a beat frequency amplier 2l supplying a beatfrequency indicator such as a pair of headphones I6.

The basic manner in which the controlled-frequency apparatus of Fig. 1operates is as follows: A crystal oscillator is made to supply twoaccurate reference frequencies by multiplication and/or division; thelocal oscillator to be synchronized is compared to the higher referenceto obtain a beat at the difference frequency and this beat is maintainedat an accurate value by suitable control circuits actuated by a devicewhich compares the beat to the lower reference. The system is inequilibrium when: (1) the frequency of the beat is a multiple of thelower reference or incremental reference, and (2) the phase of the beatsignal is in strict relationship to the phase of the lowerreference. Thephase requirement assures the desired accuracy of lock-in.

One may considerv an accurate source, such as 'a crystal, at frequencyF1, and a local oscillator to be maintained at a frequency Fz. The lowerfrequencyl reference is obtained by division of the crystal frequency bya factor N. The local oscillator can then be locked in at frequenciesdefined b y where i m='i1,i2,

For a system lin which the final output is conslderably higher infrequency than F1 the frequency F2 may be multiplied by a factor, suchas M. Then asomar Hf 'With' .the output; band A denoted :A :by E6 andthesteps Vinfoutpful', frecurencyby '5F05 we 1have.

With Fn and-AF@ prescribed-,'-N lmayibe de ter-mined With the physical*restrictionfthat `it must be obtainable bvrequency Vmultiplicationand/'or division "int-electrical" circuits.A Likewise; M is determinedwith ac'rrumberof :integral values possible through the vchoice of` F1..There' -iswno requirement thatt Fi itself; be ithe'ffundamental* crystalfrequency; maybe derived` eiectrically as an accurate multipleor'sub-multiple-of thel crystal.

It' may rbe thatdesired values of FuandfAFt donot define a value of Nwhich ispobtaina'ble electricaliy.. In this casey two crystals as in thearrangement of Figi maybe employed, one-to determineF11J and the secondto determine the increment defined' previously by the ratio N I. ThisVincreinentalf.frequency may be-:denotednby Fs.; Then, -forwthe twocrystal xcase -inFig.. 8

FofrM-Fi as before Auz'lli'i'f;VA

Certain embodiments of* the A.invention andn certain methods. ofoperation embraced therein have been shown and particularly describedfor theburpose of ennlaininglthegorinciple of operatonfofVibelinvention, andsh'owing its application', but it is-to, beunderstoodthatthe. invention contemplates suchemodicationsas ,are withinthe scopeof the appendedelaims. Y

What I .claim as new andesireto secure by Letters Patent of Y ,theUnited .States is:

l. An -,osci1latorand control unit responsive to an upper referencefrequency andan incrementalfrequency comprising Y an .adjustablefrequency oscillator, a .highefrequency :ointly ener: gized ,by theupper reference:frequencyVv and the.. oscillator -f-sr producing a heatfrequency, a pulser responsive to the .tv.incre-mentalY `Areferencefre-v, quency for producing short duration pulsesof the same. frequencyVas. kthe...incrementalfrequency,.a beat frequency mixerlfor comparing.saidbeat -frequencyand said pulse output, a dis-,1 criminatorresponsivefto the beatzfnequencymixen forzprodueing; unidirectonaloumutresponsive to the phase relationshipr betWeenf-the beat frequency signaland the pulse signal, a reactance tube connected in controllingrelationship to said oscillator, and a control connection to saidreactance tube energized by the output of said discriminator.

2. In combination with a variable phase input source. a referencevoltage;source, a beat-frequency detector, means for supplying voltagesfrom; said` sourcesfto said detector., `an incremental referencefrequency source; means .for converting the incrementalreferencefrequencv into a rectangular reference Wave, means .forcombining the rectangular wave and the beat frequency output of .the`beat frequency detector, and a discriminator*responsiveito-saidcombined output for producing a unidirectional voltage responsive to thephase-relationship.between:the rectangular `Wave and thebeat-frequencyoutput:

3. A synchronizer having inputwconneutions from the output 'of agenerator,y from `an upper reference frequency Hsourceand from animare-Jmental frequency source, said synchronizercompris-ing'meansfor-producing'a pulse yvavelsyn-JV chronized with the incremental referencecifre. quencyv source, a beat frequency1 detector Ajointly responsivetothe generator output and the upper: reference 'frequency for' producingVa fbeat frei-A quencyymeans for additively combining the beat frequencyfand the .pulse "wave, means for .difierentia'lly combining .the.beatfrequency'andpiilse` Wave; #diierentially connected'. unilateralvyele@ mentsaespectively 4responsive'to the additive and dierentialcombination. of said beat frequency: and; pulse Wave, wheneby-:theresultant output'o said unilateral-"elements representa-tive ofphasefrelationship 1 between Athe generatorr'foutput; and the. upperreference requencyi 4; 'The method 7ofcomparingthe phasefmelaeitionshipbetvveenV an oscillator adjustable `in aphaseand frequencywithz.a;,reerence ,WavepandLan iii-L cremental frequencyv wave',vwhichnnethodz ccm-A oscillator outputfand :th-e reference A:Waveoutput:Y

5. The meth odof synchronizing- 'anl oscillator witlof ai desired ffnequency .deviatingfby a* xeddifef ference rk'frequency from a standardfrequency:

source,- I which vmethod :comprisesfsu'btlividing the frequencyiof thestandard frequency'source.

such a ratio as to produceva, subdivided standard, freq-uencyeto whichthe difference frequencybeazs:y an integral ratio.,A producing fapuisewave-having@ shortfduration tpulses-:synchronizedwith thesub-z dividedfrequency; beating the oscillator output-4 with the standard frequency.to produce `a beat.

frequency; .andcomparing'thephaserelatonship betweenthe.beatfrequencyand Athe pulse -Wavaf 6. Themethod of synchronizing"anoscillatory:- of the type-havingatankcircuit; with awdesirerlvfrequencyfdeviating Aby. a xedfdiierence frequency: fromM v.astandard1xfrequermy source, 'whichvv method. comprises.' sul-:dividingthe `frequency,v of', thevv standard,frequencysourceinzsucha ratio as.to4 uproduce: a subdivided: :standard frequency to whichtthe 'idiierencelfrequency. bears an mtegmzl:

ratimprorucingfa pulse Awave having sl'iortid-unas tion pulsessynchronous with-the subdivz'uzled:free@` quency, beating-theoscilla-tor output withthe4 standard frequency to produce a-:beatfrequency,

comparing the aphase relationship between tine lmeatl frequency. .andthe. pulse wave; applyingaf quadrature eurrentitocthe tank .circuit and.vary-wfing vthe magnitude of the quadratureeurrent 4arfv cordingWithvariatons in the-*phasebetween. the" pulse 'Wave and they beatfrequency Wave.

7.111 `combination areference` frequency:source--4 having a pairoffoutputvchannels, `a frequency.

changerH linterposedV one 'of' said channelsfa a discriminator jointlyresponsive to said pulser and said beat frequency to produce a controlvoltage for adjusting the frequency of said oscillator.

8. In .an oscillation frequency controlling de vice: a source ofpredetermined frequency signal; a pulse forming device coupled to saidsource for producing pulses at times when signals from said source arein a particular phase; a second source of signals of predeterminedfrequency, the freu quency of said second source being higher than thatof the rst said source; .an oscillation generator, the frequency ofwhich is to be controlled: means for mixing signals from said generatorand from said second source to obtain heterodyne signals; and frequencycontrolling -means responsive to a change in relative phase between thesignals from said pulse former and the signals from said mixing means;said frequency controlling means being operatively connected with saidoscillation generator to control the frequency of the oscillationsgenerated therein in response to a change in said relative phase.

9. In an oscilla-tion frequency controlling device: a source ofpredetermined frequency signal; a pulse forming device coupled to saidsource for producing pulses at times when signals from said source arein a particular phase; a second source of signals of predeterminedfrequency, the frequency of said second source being higher than that ofthe first said source; an oscillation generator, the frequency of whichis to be controlled; means for mixing signals from said generator andfrom said second source to obtain heterodyne signals; phase detectionmeans, responsive to a change in relative phase between the signals fromsaid pulse former and the signals from said mixing means, to provide aunidirection signal corresponding to such changes of phase; andfrequency controlling means responsive to said unidirectional signals;said frequency controlling means being operatively connected with saidoscillation generator to control the frequency of the oscillationsgenerated therein in response to a change in said relative phase.

ROBERT C. MOORE.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,985,046 Morrison et al Dec. 18,1934 2,178,074 Jakel et al Oct. 31, 1939 2,201,978 Bedford May 28, 19402,209,507 Campbell July 30, 1940 2,234,830 Norton Mar. 11, 19412,250,284 Wendt July 22, 1941 2,288,025 Pomeroy June 30, 1942 2,312,079Crosby Feb. 23, 1943 2,354,800 Deal Aug. 1, 1944 2,358,454 GoldstineSept. 19, 1944 2,369,663 Dennis et al Feb. 20, 1945 2,406,125 Zeigler etal Aug. 20, 1946 OTHER REFERENCES Publication No. ST-28, "Cathode RayTubes and Their Applications by Stinchileld, RCA Radiotron Co., Inc.,Harrison, N. J., December 1934.

